The present invention relates to an architecture for implementing video conferencing through personal computers (PC's) or workstations over a network.
An approach to implementing video conferencing employs an architecture that is similar to that which cable TV companies use to distribute their TV signals to subscribers. Each user workstation that wishes to transmit over the network is assigned a channel from among a limited number of available channels, e.g. channels 2-50.
For a video conferencing network to be practical, the video signal from any given workstation must be able to reach all other workstations on the network, regardless of how far they are from the sending workstation. To accomplish this, some video distribution systems use what is referred to as a mid band split. According to this approach, the available channels are divided into two groups, namely, a low group (e.g. channels 2-25) and a high group (e.g. channels 26-50). Each workstation on the network sends its video signal to a distribution unit (also referred to as a head end unit) located elsewhere on the network. The head end unit receives all workstations signals transmitted on the low group channels, shifts them up to a corresponding high group channels, and then sends them back out over the network. For example, the head end unit may shift channel 2 up to channel 26, channel 3 up to channel 27, etc. The high group channel signals that are broadcast by the head end unit are periodically boosted by amplifiers that are spaced along the length of the network.
In such a network, each workstation sets its output signal level high enough so the signal will reach the head end unit with sufficient strength. The head end unit on the other hand sets its broadcast signal strength based only on how far the nearest boost amplifier is from it. In addition, the boost amplifiers are spaced such that signal strength at the output of an amplifier will not exceed some maximum acceptable level. In this way, the boost amplifiers if properly located make sure that all workstations can receive sufficiently strong signals while at the same time the workstations that are closest to the head end unit (or an upstream boost amplifier) are not overwhelmed by the signal strength of the broadcast signal.
This approach has a number of drawbacks. Since the head end unit is relatively expensive, the up front costs associated with installing such a system can be sizable. In addition, signal quality tends to degrade significantly as the network grows in physical size and in its number of users. Each of the signals converging on the head end unit from the workstations adds noise to the broadcast signal. In addition, each boost amplifier adds noise to the signal when it amplifies it. Thus, the farthest workstations from the head end unit suffer the most in signal degradation.